No one was injured when the cowling for the right engine on Flight 409, headed for Fort Lauderdale, Fla., became detached and the flight crew had to return to O'Hare a little after 5:30 a.m., said Misty Pinson, spokeswoman for the Florida-based airline.

About 5:45 a.m. after the plane took off, a passenger saw that the engine cowlings on the right side of the Airbus A319 were missing, FAA spokeswoman Elizabeth Isham Cory said. The crew "received an indication of a possible mechanical issue," and declared an emergency, Pinson said in an email.

One passenger who took pictures of the engine after the cowlings fell off said he didn't at first know what was going on. He heard "No loud noises or anything."

When flight attendants made vague announcement saying the pilots were aware of what was happening, the passenger, Chad Musolf, "thought someone was sick. Didn't take too long to realize what happened after glancing out the window."

The plane returned to O'Hare and landed soon after, Cory said.

The plane was brought back to the gate, and customers were able to leave the plane safely with crew help, Pinson said. Those who wanted to were able to get on another plane that headed to Fort Lauderdale, while those needing to make connections were rebooked. Customers also were offered refunds if they preferred.

"Spirit seemed to do a pretty good job getting all the passengers rerouted, especially those with connections," said Musolf. "That said, it took them a while to figure everything out after we got off the crippled aircraft ... A lot of passengers were confused and frustrated."

The Chicago Fire Department declared an aircraft standby response for the incident, which includes six ambulances being dispatched, but no one was injured, a fire department spokesman said. The right engine cowlings were found on airport property, and Spirit crews were inspecting the plane and the cowlings, Pinson said.

The FAA was to take possession of the cowlings as part of its investigation of the incident, Cory said.